Hasan ben Mar Hasan, also known as ʿAlī ibn Mar Hasan was born somewhere in al-Andalus in the tenth century. He was dayyan (judge) of Cordova in the late tenth century. He wrote three books on the calendar and intercalation that are no longer extant, but which are referred to in the writings of Isaac Israeli (YesodʿOlam), Abraham bar Ḥiyya (Seferha-ʿIbbur), and Abraham ibn Ezra (Seferha-ʿIbbur). The date he used in his astronomical calculations (972), in which he followed the system of the Arab astronomer al-Battānī (d. 929), is the only certain reference we have to date his lifetime.

Josefin…